This is highly specific to COM interop code and C# language version 4 and up. Yes, it permits omitting ref in this specific case. C# version 4 had a lot of tweaks to make Office programming easier. The Office api was originally designed to work well with early VB versions, they used ByRef by default. Still the case for VBA. So the api has a lot of method parameters that are ByRef, even though they don't actually modify the passed argument.

The compiler will generate a temporary variable if necessary to make the call legal. With the consequence that your variable doesn't get updated. Something you'll have to watch out for. Two steps forward, one step back :)